1. Field of Invention
The present invention relates to a phase-shifting mask structure. More particularly, the present invention relates to a phase-shifting mask structure that produces improved alignment and an increased precision in alignment checking.
2. Description of Related Art
In photolithographic processing, the maximum resolution provided by exposure is closely related to the wavelengths of the light source and the depth of focus (DOF). When a shorter wavelength light source is used, or when the depth of focus is increased, the resolution obtained is better. However, when the wavelength of the light source is shorter, the depth of focus will be smaller. That is, the two have a reciprocal relationship. Therefore, in the design of an exposure system, a certain trade-off must be made to get the highest resolution from the system. In general, the most common wavelengths for the light source in photolithographic processing operation include two main types, the so-called g-line and the i-line. The former has a wavelength of 4460 .ANG., which is in the ultra-violet region, while the latter has a wavelength of 3650 .ANG.. Currently, even deep ultra-violet light that has a wavelength of 2480 .ANG. is sometimes used as the light source in the exposure.
Besides using shorter wavelength light as the light source, another method to increase resolution is to use a phase-shifting photolithographic technique. A phase-shifting technique is similar to a conventional one. The only difference is the addition of a phase-shifting layer over all of the photomask. Through the negative-positive interference between the photomask and the phase-shifting layer, the pattern projected onto the wafer by the projector has a higher resolution.
In photolithographic processing, the tools necessary for the operation, besides a light source, also include a supply of mask for the transfer of circuit patterns. The main body of the mask is a piece of flat and transparent glass, including a layer of chromium having a thickness of several hundred angstroms disposed over the surface. To minimize reflection during exposure, sometimes an additional layer of chromium dioxide is formed above the chromium film. In order to increase the resolution of transferred patterns, patterns are transferred by a stepper in a step-and-repeat fashion such that various mask patterns are transferred one-by-one in many exposures to a wafer.
When various mask patterns are exposed layer by layer on top of the wafer, the top and bottom mask patterns must be perfectly aligned before and after the exposure. As shown in FIG. 1, one way to ensure that the patterns are properly aligned is to make use of alignment slits 16 and alignment-checking patterns 18 on the peripheral region 12 around the pattern zone 14 of a mask plate 10. In practice, the alignment slits 16 are first checked to make sure the mask pattern is properly aligned before the exposure is performed. Then, the alignment-checking patterns 18 are used to countercheck the precision of alignment after the exposure.
FIG. 2A is a magnified view of the alignment-checking patterns 18 in FIG. 1. The alignment-checking patterns 18 include patterns 20 and complementary patterns 26 of light-blocking thin films, and corresponding patterns 22 and complementary patterns 24 of light-penetrating thin films. However, the light-blocking thin films do not block all in-coming light for the coventional phase shift mask. FIG. 2B is a magnified view of the alignment slits 16 in FIG. 1. The alignment slits 16 include a set of parallel strips 260 and a complementary pattern 200 of light-blocking thin films and a corresponding pattern 240 and a set of complementary parallel strips 220 of light-penetrating thin films. Again, the light-blocking thin films do not block all in-coming light.
In general, the light-blocking thin films of the alignment slits and the alignment-checking patterns on a phase-shifting layer used in a conventional process do not have absolute light-blocking power. In other words, the light-blocking thin films are actually semi-transparent to light. If such semi-transparent layers are used to carry out exposures and then develop on the wafer surface, the fiducial mark thus established is likely to be distorted and shifted. This will lead to alignment errors of the alignment checking system. Furthermore, since the patch of alignment profile left on the wafer is going to be fuzzy, subsequently laid mask patterns will be slightly misplaced too.
In light of the foregoing, there is a need in the art to improve the phase-shifting mask structure.